The present invention relates to a charge/discharge device, and to a power supply device for a charge/discharge device or the like. In particular, it relates to a power supply device, to the output terminals of which a secondary battery is connected, and which is adapted for performing testing of the battery characteristics (the electrical discharge characteristic and the capacity and so on) of this secondary battery by repeatedly performing charge and discharge thereof.
In the testing of a secondary battery such as a lithium ion battery or the like, normally, testing is performed by repeatedly charging and discharging the secondary battery by connecting it as a test subject to output terminals. For example control may be performed so as, when the battery voltage of the secondary battery is low, to establish a charge mode and flow a charging current into the secondary battery, and so as, when the battery voltage of the secondary battery is sufficiently high, to establish a discharge mode and flow a discharge current out of the secondary battery.
With this type of power supply device, a DC power supply unit is connected to the input side, and during the charge mode a charging current flows from the DC power supply unit to the secondary battery, while during the discharge mode a discharge current flows from the secondary battery to the DC power supply unit. Moreover, a series circuit of a first switching element and a second switching element is connected in parallel to the DC power supply unit, and a smoothing filter circuit, which comprises a series circuit of an inductor and a smoothing capacitor, is connected in parallel to said second switching element.
Furthermore, there is also provided a drive circuit which generates first PWM pulses and second PWM pulses for respectively turning said first switching element and said second switching element alternatingly ON and OFF, and which also generates a dead time between those PWM pulses (for example, refer to Japanese Laid-Open Patent Publication 2002-10502).
The power supply device having the above structure operates as follows.
When a secondary battery which is not charged is connected to the output terminals, the pulse width of the first PWM pulses becomes longer than the pulse width of the second PWM pulses, and the average voltage between the two ends of the second switching element becomes higher than the battery voltage of the secondary battery. Due to the difference between this average voltage and the battery voltage of the secondary battery, a charging current flows into the secondary battery.
In this charge mode, the charging current (i.e. the output current) is detected by an output current detection unit, and the duty ratios of the first PWM pulses and the second PWM pulses are controlled so that this current is kept at a constant value. Furthermore, a dead time is generated so that said first switching element and said second switching element are not turned ON at the same time, and thus the first PWM pulses and the second PWM pulses are not both turned ON together.
When the voltage of the secondary battery becomes sufficiently high, the system changes over to the discharge mode. In this discharge mode, the pulse width of the second PWM pulses becomes longer than the pulse width of the first PWM pulses, and the average voltage between the two ends of the second switching element becomes lower than the battery voltage of the secondary battery. Due to the difference between this average voltage and the battery voltage of the secondary battery, a charging current flows from the secondary battery towards the DC power supply unit.
In this discharge mode, the discharge current is detected by the output current detection unit, and the duty ratios of the first PWM pulses and the second PWM pulses are controlled so that this current is kept at a constant value. Furthermore, the dead time is set so that said first switching element and said second switching element are not turned ON at the same time, and thus both the first PWM pulses and the second PWM pulses are not both turned ON together.
By doing this, charging and discharging of the secondary battery are repeated, and capacity testing and other types of testing of the secondary battery may be performed.
With the power supply device described above, according to the progress of charging or discharging, the pulse widths (i.e. the ON widths) of the first PWM pulses and the second PWM pulses, or the OFF widths of these pulses, are gradually narrowed down progressively. For example, in the discharge mode, when discharge of the battery progresses and the battery voltage becomes low, control is exerted so that the OFF width of the second PWM pulses becomes shorter, in order to turn the second switching element ON for a greater proportion of the time.
However, when the dead time between the first PWM pulses and the second PWM pulses is formed by a circuit which includes a CR time constant circuit which can be built at low cost, the following problem arises.
That is, such a CR time constant circuit is a circuit which creates a delay by taking the edge off the pulse waveform, but, due to the analog signal processing, in a control state in which the OFF width of the second PWM pulses becomes shorter, the formation of these second PWM pulses becomes unreliable, and this control becomes unstable, which is undesirable. Even if, alternatively, the dead time is created by performing the waveform processing in an analog manner, the same problem arises. On the other hand if, in order to solve this problem, the first PWM pulses are formed perfectly in a digital manner, then the drive circuit for creating the dead time becomes a high price digital circuit, and a DSP or a CPU of high performance and high cost becomes necessary for forming the switching pulses, so that an overall increase of the cost of the power supply device is entailed.
The above describe type of problem also occurs in a similar manner in the charge mode as well. Furthermore, even if this power supply device is made to function as an electronic load device which can vary the magnitude of the load from the point of view of a secondary battery which is connected to its output terminals, the same types of problem as described above still occur.
Thus, the object of the present invention is to provide a power supply device for a charge/discharge device or the like, which is capable of preventing control becoming unstable in a state in which control is being exerted so that the pulse width of the first PWM pulses or the pulse width of the second PWM pulses becomes short, even without providing a high cost digital circuit or the like to the drive circuit.